Multiple tissue gases can be monitored using a diffusion membrane tipped probe coupled to a mass spectrometer. The tissue with probe in place constitutes a system in which gas tension readings are a function of many factors including local diffusion and convection processes and other characteristics of the organ or tissue being studied, as well as the material and geometry of the diffusion membrane probe. However, the interrelationship of these factors is only partially understood and there is no experimentally verified quantitative model of tissue appropriate to the description of the tissue-probe system. Such a description may permit an expanded diagnostic role of the diffusion membrane probe in evaluating abnormal tissue function. Recently, we have found experimental evidence in support of a compartmental mental for the tissue-probe system and have also found that local subcutaneous tissue perfusion relates linearly to a depletion coefficient derived from diffusion membrane probe readings. The study planned here is to further elucidate these results and to develop a model tissue which interrelates parameters such as tissue perfusion, oxygen consumption, tissue diffusion, and tissue and blood volumes, with mass spectrometer readings from the tissue-probe system. Experimental data will be obtained from the dog hind limb during control conditions, following changes in arterial gas concentrations, during arterial and venous clamping, and during pump perfusion of the limb. Tissue gases including oxygen and carbon dioxide and inert gases such as argon will be analyzed using teflon and silicone tipped mass spectrometer probes. Additionally, arterial and venous blood will be sampled and analyzed for gas tensions and pH.